Background of the Invention
[0001] This invention relates to blood analyzers used to measure various components in a
blood sample, for example in medical diagnosis and research.
[0002] The ratio of the volume of packed red blood cells from a whole blood sample to the
total sample volume is a useful measurement for diagnosing anemia and other disease
conditions. That ratio usually is referred to as the "hematocrit ratio" or the "hematocrit
value", and it is usually determined by centrifuging a whole blood sample to separate
cells from plasma. It is known that, all other things being constant, the conductivity
of a blood sample varies as a function of its hematocrit value, but other blood components,
notably electrolytes, influence conductivity significantly, and the conductivity of
those components must be accurately accounted for if a reliable hematocrit value is
to be derived from conductivity readings.
[0003] Automated equipment for determining blood components, such as electrolyte concentration
or dissolved blood gas partial pressures, often involve the use of electrodes positioned
along a flow path. When whole blood is introduced in the flow path, the electrodes
provide a reading of the desired blood characteristic. Currently, electrodes are available
to provide electrical signals representative of various blood components such as sodium
ion concentration ("[Na
+J"), potassium ion concentration ("[K+]"), calcium ion concentration ("[Ca
++]"), hydrogen ion concentration ("pH"), partial pressure attributed to 0
2 ("P0
2"), and partial pressure attributed to C0
2 ("PC0
2"). From time to time it may be necessary to replace various components of a blood
analyzer flow path, such as an electrode or a rubber inlet septum. Moreover, particularly
in analyzers with a small, tortuous flow path having dead spots, whole blood may clot,
resulting in lost time from shut-down, disassembly, cleaning, re-assembly, and re-starting
of the apparatus.
Summary of the Invention
[0004] In one aspect of the invention, the hematocrit level of a blood sample is measured
by flowing the sample along a liquid flow path and using means in the flow path to
obtain electrical signals representative of the sample's electrical conductivity and
of the concentration of an ion species in the sample. Standardizing solution is introduced
in the same flow path, either before or after the sample measurement. The standardizing
solution has a known ion species concentration and a conductivity indicative of a
known equivalent hematocrit value; "equivalent" hematocrit value is used in this application
to mean the hematocrit level of a blood sample having a conductivity corresponding
to that of the standardizing solution, even though the standardizing solution contains
no whole blood cells and has an actual hematocrit value of 0. Electrical signals are
obtained for standardizing solution conductivity and ion species concentration. A
tentative sample hematocrit value is derived from the sample and standardizing conductivity
signals, with reference to the known equivalent standardizing hematocrit value. Then
the tentative hematocrit value is corrected with reference to the sample and standardizing
ion concentration signals and to the known standardizing solution ion concentration
value.
[0005] In preferred embodiments of the method, an external validation of the apparatus is
provided from time to time by introducing a control solution into the flow path, which
is described below in connection with the third aspect of the invention. Also in preferred
embodiments, conductivity of solutions in the flow path is obtained by: a) providing
electrodes in the flow path coupled to a constant current AC circuit via a transformer;
b) applying an AC signal from the AC circuit to the electrodes via the transformer;
and c) detecting reflected impedance in the AC circuit. The method comprises: a) obtaining
the electrical signals representative of standardizing conductivity and standardizing
ion concentration; b) storing signals representative of the known standardizing equivalent
hematocrit value and the known standardizing ion concentration value; c) obtaining
the electrical signals representative of sample concentration and standardizing ion
concentration; d) comparing the sample and the standardizing ion concentration signals
with reference to the stored known standardizing value signal, to derive a signal
representative of sample ion concentration value; e) comparing the sample and the
standardizing conductivity signals with reference to the stored standardizing hematocrit
value signal to derive a signal representative of a tentative sample hematocrit value;
and f) correcting the tentative sample hematocrit value signal with reference to the
sample ion concentration signal and the stored standardizing ion concentration value
signal. Preferred ion species for use in the method are Na
+ or Cl
-.
[0006] The invention also features, in another aspect, apparatus for determining hematocrit
value in a blood sample comprising: 1) means for providing a fluid flow path; 2) means
in the flow path for providing an electrical signal representative of the conductivity
of liquid passing along the flow path; 3) means in the flow path for providing a signal
representative of the concentration of an ion species in liquid passing along the
flow path; 4) means for introducing the blood sample into the flow path to obtain
a signal representative of sample conductivity and of sample ion species concentration;
5) means for introducing into the flow path a standardizing solution having a known
concentration of an ion species and having a conductivity representative of a known
equivalent hematocrit value; 6) means for deriving a signal representative of a tentative
sample hematocrit value from the sample conductivity signal, with reference to the
standardizing conductivity signal, and to the standardizing equivalent hematocrit
value; and 7) means for correcting the tentative sample hematocrit value with reference
to the standardizing and sample ion concentration signals and to the known standardizing
ion concentration.
[0007] In preferred embodiments the apparatus includes: a) means for storing either the
sample or the standardizing conductivity signal, and means for comparing the conductivity
signals with reference to the known standardizing equivalent hematocrit value to generate
a signal representative of the tentative sample hematocrit value; and b) means for
correcting the tentative sample hematocrit value signal including means for storing
either the standardizing or the sample ion concentration signal and comparing the
ion concentration signals with reference to the known standardizing ion concentration
value. The apparatus comprises at least two standardizing solutions, each of which
has a conductivity indicative of a known equivalent hematocrit value and a known ion
concentration. The conductivity measuring means comprises electrodes in the flow path,
a constant current AC circuit coupled to the electrodes via a transformer and means
for detecting the reflected impedance in the AC circuit. Specifically, the conductivity
signal-generating means comprises: 1) a first transformer for coupling the AC circuit
to the electrodes; and 2) a second transformer for maintaining constant current in
the AC circuit; means establishing a loop between the electrodes and means, connected
in the loop between the second transformer and the electrodes, selected to compensate
for inherent capacitance at the electrode/sample interface. The apparatus comprises
an ion species sensing electrode positioned in the flow path and connected via an
electrical output circuit connected to the input of a multiplexer, the impedance detecting
means also being connected to the input of the multiplexer, the multiplexer having
an output means connected via an analog-to-digital converter to a means for storing
and comparing signals, and to the means for correcting sample conductivity.
[0008] In a third aspect the invention features a control solution kit for evaluating the
hematocrit detection apparatus. The solution comprises an aqueous solution of the
ion species (e.g. Na
+ or Cl-) at a known concentration; and an ion activity enhancing agent (e.g. a polyol
selected from glycerol and polyalkyl .glycols). The solution has a conductivity representative
of a known equivalent hematocrit level, and both the ion concentration and the equivalent
hematocrit value preferably are within physiological ranges (e.g., [Na
+] is between 130-150 mM, and hematocrit is between 40 and 55%).
[0009] The hematocrit measurement aspects of the invention provide rapid, accurate highly
automated measurements of the hematocrit level, without the need for the analyzer
user to store whole blood standards .
[0010] Other features and advantages of the invention will be apparent from the following
description of the preferred embodiment and from the claims.
Description of the Preferred Embodiment
[0011] We first briefly describe the drawings.
I. Drawings
[0012]
Fig. 1 is a front view of a blood analyzer.
Fig. 2 is a diagrammatic representation of the fluid flow path and some of the electrical
components of the analyzer of Fig. 1.
Fig. 3 is a side view, in section, of the septum assembly and septum mounting plate
of the analyzer of Fig. 1.
Fig. 4 is a view along 4-4 of Fig. 5.
Fig. 4A is a section of a septum from the septum assembly of Fig. 3.
Fig. 5 is an exploded view, with parts broken away, of the septum assembly and mounting
plate of Fig. 3.
Fig. 6 is a side view of the electrode holder assembly of the analyzer of Fig. 1.
Fig. 7 is a view, in section, along 7-7 of Fig. 6.
Fig. 8 is a side view of the holder assembly of Fig. 6 with parts exploded, broken
away, and in section.
Fig. 8A is a view of the reference block of the assembly of Fig. 6, taken along 8A-8A
of Fig. 8.
Fig. 9 is a view along 9-9 of Fig. 8 with parts broken away and in section.
Fig. 10 is a view, in section, along 10-10 of Fig. 8.
Fig. 11 is a plan side view of an electrode clip for use in the assembly of Fig. 6.
Fig. 12 is a diagrammatic representation of electronic components and functions related
to the hematocrit detector of the analyzer of Fig. 1.
Fig. 12A is a graph of the reciprocal of resistivity versus l/(l-hematocrit value).
Fig. 13 is a diagrammatic representation of the electrical functions of the analyzer
of Fig. 1.
II. Structure
[0013] Analyzer 10 of Fig. 1 provides for measurement of the concentrations of certain electrolytes
and gases in a small (e.g. less than about 0.25 ml) sample of whole blood that has
been treated (e.g. with heparin) to prevent coagulation. Specifically, the treated
sample is drawn from its container through a probe 20, along a sample flow path, and
out a waste outlet 28 (Fig. 2). Readings of sample P0
2, PC0
21 [Na
+], [K
+], [Ca
++], and pH are provided on a C.R.T. display 12 and a tape printer 14. The same flow
path includes means to provide a measurement and readout of the sample hematocrit
value.
[0014] The above measurements are performed as described in greater detail below, using
electrodes and associated components that yield an electric signal representative
of the characteristic being measured. In order to ascribe a value to the signal, the
electrodes are standardized periodically with standard gases from replaceable cylinders
and with standard fluids from a replaceable fluid pack 18 whose components and operation
are also described below. The operation of the electrodes and standardizing apparatus
is controlled by a computer 130 (Fig. 13) in response to a control program and to
the operator's entries on keypad 16.
A. Sample Flow Path
[0015] As illustrated in Fig. 2, probe 20 is a hollow elongated metal tube (e.g. stainless
steel) having a fluid inlet 21 at one end and connected at the other end to a fluid
flow path. A probe drive motor 22, controlled by controller 222, moves the probe longitudinally
through septum assembly 24, while the probe outlet remains in communication with the
fluid flow path. The furthest longitudinal extension of the probe in the direction
of arrow A is shown in Fig. 2, with probe inlet 21 positioned outside the septum assembly,
immersed in a sample 26 that is to be drawn through the inlet and along the flow path.
[0016] Fig. 2 diagrams the sample flow path through an electrode assembly (best shown in
Figs. 6-10 and described in greater detail below) that includes: a heater block 3d
heated by a resistance heater 160; a series of six electrodes, 31, 33, 35, 37, 39,
and 41 in an electrode block 80 that enable generation of signals representative of
P0
2, PCO
2, pH, [Ca
++], [K
+], and [Na
+], respectively; and a reference block 105. The external, mechanical configuration
of the electrodes is described below; the electrochemical principles and composition
of the electrodes are conventional. From electrode block 80, the sample flows to waste
outlet 28. The fluid flow is drawn along the path by a peristaltic pump 29, driven
by stepper motor 230 under the control of controller 229.
[0017] Along the flow path, there are air detectors to sense conductivity changes representative
of the change from air to liquid, thereby providing an indication of air/liquid transitions
and thus to signal changes from one fluid to another and to verify sample and standard
positioning. Specifically, one air detector 32 is positioned in heating block 30,
and a detector 69 located in heater block 30 serves as a hematocrit level detector
as described in greater detail below. A third air detector 103 is located in the electrode
block. Finally, a clamp electrode 43 is positioned upstream from waste outlet 28 to
connect to circuitry that minimizes the common mode voltage range and thereby improves
the sensitivity and stability of the electrode measurement.
B. Standard Flow Paths
[0018] The analyzer has been designed particularly to flow the various standard fluids through
the flow path and to flush the flow path, while minimizing any opportunity for contamination
between standards, or between a standard and a blood sample. As best shown in Fig.
2, the standards are assigned to specific flow paths and chambers in septum assembly
24, and from there, the standards flow through the above-described sample flow path
to waste outlet 28. The various standards and their flow paths are:
1) GA, which is a source of gas having known PO2 and PCO2 composition, connected via metering solenoid valves 46 (sold by Lee Company, Westbrook,
Conn.) to a humidifier 47 and thence, via line 48 to chamber 49 of the septum assembly
24.
2) GB, which is similar to GA, having different P02 and PC02 composition, thereby enabling standardization of those two electrodes; GB communicates with chamber 49 of septum assembly 24 via solenoid valves 46', humidifer
47' and line 48'.
3) pHA, a liquid of known pH that flows via line 53 to chamber 54 of septum assembly 24;
4) pHB, a standard similar to pHA, having a pH different from that of pHA, that flows via line 53' to septum assembly chamber 55. Standard pHB has a total conductivity indicative of a known equivalent hematocrit value. As explained
in greater detail below, a solution having a known conductivity can be treated as
the equivalent of a whole blood sample having a specific "equivalent hematocrit value."
5) EA, an electrolyte standard having a known [Na+] , [K+] , and [Ca++] and also having a total conductivity indicative of a known equivalent hematocrit
value different from the value of pHB; EA flows via line 56 to septum assembly chamber 57.
6) E , an electrolyte standard having a known [Na+] , [K + ], and [Ca++], different from those of EA; standard EB flows via line 58 to septum assembly chamber 60.
The composition of the various standard solutions is given in more detail below.
[0019] Each of lines 53, 53', 56, and 58 flows through a pinch valve 51 that is controlled
by D.C. motor 63, and controller 64 to shut those lines selectively and separately
when they are not in use. Each of lines 53, 53', 56, and 58 flows through a preheater
to warm the standard solutions somewhat before they enter the heating block 30. A
flush line 61 bypasses pinch valve 51 and flows through preheater 52 to septum assembly
chamber 62. Lines 48, 48', 61, 56, 58, 53, and 53' terminate in a rigid multi-plug
connector 161 that is adapted to cooperate with the septum assembly 24 so that all
of the lines can be connected simultaneously. Specifically, connector 161 is shaped
to fit within recesses of the septum assembly surrounding each inlet to a septum assembly
chamber and, when connector 161 is properly positioned, an outlet from each of the
lines 48, 53, 53', 56, 58, and 61 removably seals to the appropriate septum inlet
by overlapping it.
[0020] A high molarity reference solution (Ref) flows through line 67 where it contacts
reference electrode 34, and from there into the above sample flow path between clamp
electrode 43 and waste outlet 28. The use of an open reference junction (i.e., a junction
that is not enclosed in a membrane) enables the use of a low pressure flow for reference
solution, and thereby reduces any possibility of contamination of the sample flow
path or the electrode sensors by reference solution. The dotted line 64 indicates
the region of the analyzer bathed in air from heater 66 driven by fan 65 (connected
to controller 66' and fan-fail monitor 65') to stabilize temperature.
[0021] Three specific features of the analyzer are discussed below in greater detail: septum
assembly 24; electrode assembly 68 (Fig. 6); and hematocrit detection via conductivity
detector 69.
C. Septum Assembly
[0022] Referring to Figs. 3, 4, 4A, and 5, removable septum assembly 24 has chambers 49,
54, 55, 57, and 60, and 62 which are separated by rubber septa 70 (Fig. 3) that have
been slit to receive probe 20 and to form a seal around the probe as it is extended
through the assembly. The septum assembly enables the analyzer to automatically draw
one or more of the reference fluids along the sample flow path without contamination
of future samples. As best shown in Figs. 3, 4, 4A, and 5, the assembly includes an
end mounting unit 71 and a plurality of central septa supports 72, each of which has
a radial inlet 373 connecting with an axial central channel 74. A cylindrical rubber
septum 70 seats in a cylindrical cavity 75 of the end mounting unit 71 and each central
unit 72.
[0023] Fig. 4A shows a septum 70 in cross-section, free from the stresses it experiences
in the assembly. Specifically, very small (e.g..010") annular rims 70' around the
periphery of each side of septum 70 are designed so that, when the septum is seated,
cavity 75 having a restrained diameter, it is subjected to moderate radial squeezing
(arrow C) sealing at the ridge, so sealing is enhanced, and leakage around the probe
is reduced. In this way, the septum design provides an adequate seal without the need
for a tight fit that causes friction and wear as the probe moves.
[0024] The assembly is produced by aligning all of the units with unslit septa in place,
and an external sleeve 77 is then placed over the sub-assembly. The assembly then
is ultrasonically welded together. After ultrasonic welding, a knife is passed through
the central channels 74 to form small slits in each septum 70. Because the septa are
placed in alignment first, and then slit, the size of the slits can be minimized and
alignment is ensured, to reduce wear on the septa from repeated movement of the probe
through them, thereby lengthening the useful life of assembly 24.
[0025] As the slits in septa 70 become worn, the seal between chambers in the assembly can
be affected, and the possibility of contamination is increased, so that it is necessary
to replace the septum assembly from time to time. To facilitate removal of the assembly
from the analyzer, end unit 71 is designed to rotatably engage and disengage a spring-loaded
latch on mounting plate 163 of analyzer 10 as shown in Fig. 5. Specifically, a cylindrical
recess 76 on plate 163 the face of analyzer 10 includes two thick, resilient parallel
wires 373, spaced apart at a preset distance. End unit 71 of assembly 24 includes
two seating posts 78 that have parallel flat sides 80 positioned to fit between wires
373. Two flanges 381 of posts 78 are generally flat, with slightly rounded corners,
and define generally straight parallel grooves 82 spaced apart a distance that is
very slightly less than the distance between wires 373. To insert the septum assembly,
its end 71 is inserted in recess 76 in an initial position with sides 80 parallel
to, and positioned between, wires 373, and then the assembly is rotated in either
direction to engage wires 373 in grooves 82. At 1/8 turn, the wires are resiliently
forced apart by the shoulders of grooves 82 creating a position of instability such
that, a slight movement away from the 1/8 turn position will release the biasing force
of the wires to re-establish a stable position. At 1/4 turn from the initial position,
the wires seat in the grooves and lock the assembly in place. A 1/4 turn in either
direction releases the assembly.
[0026] Once assembly 24 is inserted, manifold connector 161 is forced into place so that
each of the various standardizing lines sealingly overlaps the proper inlet on the
septum assembly as shown in Fig. 4.
D. Electrode Assembly
[0027] The fluid flow path exiting the probe communicates with an electrode assembly shown
in Figs. 6-11. The path enters heating block 30 through inlet 101 (Fig. 10) and follows
a circuitous route through stainless steel tubing to allow heat transfer from the
heating block. Block 30 includes air detector 32 having a pair of electrodes 102 that
are spaced apart in a chamber having passivated (e.g. HNO
3 etched stainless steel) walls. 'Electrodes 102 are connected to a reflected impedance
detector that is driven by an AC source and generates a signal to be converted to
digital signal to control the probe via computer 130 (Fig. 13). From air detector
32, the fluid passes to hematocrit detector 69, described in greater detail below.
[0028] As shown in Fig. 8, the connection between heater block 30 and electrode block 80
is formed by a small piece of Tygon (TM Norton Co., Worcester,
Mass.) tubing 151 that fits over the ends of stainless steel tubing from the flow path
of each block; the Tygon tubing fits within countersinks in the respective blocks
surrounding the ends of the stainless tubing. In electrode block 80, the flow path
passes over each of electrodes 31, 33, 35, 37, 39, and 41 (Fig. 2) in sequence. Air
detector 103 (Fig. 2), which is positioned between electrode 33 and electrode 35,
operates as described above regarding detector 32. As shown in Fig. 9, the flow path
follows a zig-zag path between wells at the bottom of cylindrical electrode cavities
104 in block 80.
[0029] The downstream component of the electrode assembly is a reference block 105 which
includes clamp electrode 43 (Fig. 2) and a T connection upstream from it, connecting
to reference line 67, allowing reference fluid (Ref.) to be drawn out waste outlet
28. The reference electrode 34 in line 67 serves as a reference for electrodes 35,
37, 39, and 41, (the pH, [Ca
++], [K
+], and [Na electrodes). The two gas electrodes 31 and 33 have internal references.
[0030] The flow path has a relatively narrow diameter (e.g. 0.7 mm) and is tortuous as shown
in Fig. 9, and therefore clots may form in the path. Conveniently, heater block 30,
electrode block 80, and reference block 105 are separate units that can be disassembled
and replaced individually, as shown in Fig. 8, when it is necessary to replace one
of them or to clean a blood clot from them. Specifically, heater block 30 includes
a back plate 106 to which electrode block 80 is bolted. A lipped retainer 107 screws
into the top edge of plate 106 and grips a notch in the top of reference block 105;
and a lip 108 on the bottom of the rear face of reference block 105 engages a groove
in the top of electrode block 80. Electrical connections to the heaters and air detectors
of block 30 are made through multi-pin connector 44. Connections to the electrical
components of blocks 80 and 105 are made through male connector plugs that allow easy
separation of the units. A locator pin 152 extends rearwardly from plate 106 to guide
the electrode assembly as it is forced in the direction of arrow B (Fig. 8) into a
cooperatively shaped recess in the analyzer. A flow path inlet 109, a reference inlet
110 (Fig. 9), and waste outlet 28 extend from the assembly to be connected to tubing
in the analyzer.
[0031] It is particularly advantageous that the entire fluid flow path of the electrode
assembly (i.e. through the heater, the electrodes and the reference block) can be
readily removed and replaced in a short time, removing only two bolts. In that way,
when a part of the flow path becomes defective, the flow path can be replaced with
an alternate part and the apparatus can be restarted without taking time to cure the
defect in the original part. Thus downtime on the apparatus can be significantly reduced
merely by maintaining spare flow path parts.
[0032] Each 6f electrodes 31, 33, 35, 37, 39, and 41 is mounted on an individually replaceable
unit, one of which (electrode unit 31') is shown in Fig. 11. Electrode unit 31' consists
of an electrode-carrying cylinder 89 movably inserted through an opening 83 in the
back 82 of a clip 81. Clip 381 has a resiliently deflectable ridge 85 extending from
one end, which terminates in a latch 86 sized to engage a groove 87 in block 80. A
guide pin 88 extending from clip 381, at the end opposite to latch 85, fits in opening
45 in block 80. Cylinder 89 has a diameter small enough to fit easily within opening
83, and a compression spring 90 is seated between clip 81 and a flange on the cylinder,
thus biasing the cylinder into an electrode cavity 104 in block 80. A flange 153 on
the rear of cylinder 89 prevents the cylinder from passing through the clip opening
83. The PC0
2 electrode 31 is bonded to cylinder 89, and cylinder 89 is hollow to accommodate wiring
and (because it is a gas electrode with an internal reference) a reference electrode
that electrically connects the electrode to signal-generating apparatus via plug 91.
E. Hematocrit Value Detector
[0033] The apparatus provides a rapid, accurate hematocrit-value determination, electronically,
without time-consuming, labor intensive centrifuging and visual measurement and without
using a whole blood standard. The hematocrit value determination is based on the relationship
between a blood sample's electrical conductivity (C) and its hematocrit value (H),which
is given the expression
where C is the conductivity when H = O. The blood analyzer determines the conductivity
of the sample by obtaining a resistance signal and comparing it to resistance signals
from two reference solutions, each having a different known conductivity. The analyzer
includes electrical components to provide a linear signal-to-resistivity relationship
in the area of interest, so that the two references are sufficient to establish a
value corresponding to the sample resistivity signal.
[0034] The electrical conductivity of a blood sample depends on a number of factors in addition
to the hematocrit value, notably concentrations of various electrolytes, so any conversion
of standard fluid conductivity to hematocrit value necessarily implies concentration
levels for such electrolytes. The sample electrolyte concentration may vary enough
from those implied standard concentrations to require correction; however, it has
been found that, if the sodium concentration implied in the standard is used to correct
the actual sample conductivity, the hematocrit value obtained will be accurate within
the ranges necessary for blood hematocrit measurements.
[0035] In general, assuming a given [Na
+] level and given detector geometry, the resistance (R
X) is related to hematocrit value as shown in Fig. 12A, where R
0 is the resistance at H=0. Thus, R
x can be used to obtain the hematocrit value (H ) of a blood sample using the known
resistance (R
A) and known hematocrit value (H
A) of a standard A by the following equation:
where R is the resistance at H = O.
[0036] In order to determine R
o, a second standard having a known equivalent hematocrit value (H
B) is needed. One of the pH standards, e.g. pH
B, is preferably used for this purpose. By measuring the resistance (R
B) of pH
B and the resistance (R
A) of E
A, R
o can be determined from equation (2). Once R is known, and Rand R
A can be measured, and the sample hematocrit (H
X) can be obtained by rearranging equation (2), H
A being known also:
The equivalent hematocrit values of the standards can be determined by standardizing
them to actual whole blood standards.
[0037] To correct for variations in resistance attributed to variations in [Na
+], the true sample hematocrit value (H
x*) can be obtained from H using the following relationship:
where Na STD is the [Na
+] in standard E
A and Na
x is the sample [Na ].
[0038] When operating the analyzer, it is highly desirable to use an external control to
confirm the accuracy of the instrument. The external control could be a whole blood
sample having very precisely known electrolyte, pH, blood gas and hematocrit levels.
However, whole blood is relatively expensive and difficult to handle because it has
a short shelf life and is relatively unstable.
[0039] For this reason, it is desirable to use a surrogate solution that mimics whole blood
sufficiently to serve as a satisfactory control. A stable aqueous buffer having known
electrolyte and pH could serve as a control for all readings other than hematocrit.
The difficulty in using such a buffer as a hematocrit level control lies in the fact
that, at normal physiological ranges, the sodium ion concentration is about 130 mM
- 150 mM. The conductivity of such a solution provides an equivalent hematocrit value
of less than 5%, which is far below the normal range of around 50%.
[0040] It is highly desirable to have the equivalent hematocrit value of the control in
normal ranges, in part because of the limitations on the linear signal-to-resistance
range of the analyzer circuitry. One could try to raise the equivalent hematocrit
level of the control by reducing its [Na
+], but in so doing, the [Na
+] would have to be drastically reduced and therefore the correction required by equation
(4) would largely counterbalance any effective increase in the corrected hematocrit
value.
[0041] This dilemma is resolved by adding an ion activity coefficient enhancer to the aqueous
control solution in order to increase the ion activity measured by the [Na
+] sensing electrode and to increase the resistance measured by the hematocrit resistance
detector. By including such an enhancer in the control solution, the actual [Na
+] may remain well below physiological levels, but the [Na
+] sensing electrode measures ion activity, and the increased Na
+ activity coefficient resulting from the presence of the enhancer will provide a signal
equivalent to a physiological [Na
+]; thus, the [Na
+] correction resulting from equation (4) will not affect the control hematocrit significantly.
[0042] Suitable activity coefficient enhancers are polar, water-miscible organic compounds,
particularly polyols such as polyethylene glycol, glycerol, and polypropylene glycol.
It is possible, using such activity enhancers, to formulate control solutions with
[Na
+] in the normal range (130 mM - 150 mM) and with conductivities characteristic of
a sample having a normal hematocrit (40% - 55%).
[0043] Suitable control solutions have a [Na
+] of 20-60 mM, [K
+] of 0.5 mM - 1.7 mM, [Ca
++] ] of 0.1-0.5 mM, pH of 6.8-7.6 and between 10% and 50% (V/V) of an enhancer such
as glycerol. Two specific such control solutions are:
[0044] Suitable pH standards are buffered solutions exemplified by the following:
[0045] Suitable electrolyte standards are exemplified by the following:
[0046] Suitable gas standards have between 0-25% 0
2 and 0-15% CO
2, the balance being N
2.
[0047] Suitable Ref. and flush solutions are well known to those in the art.
[0048] Referring to Fig. 12, as a solution passes through hematocrit detector 69, the resistance
between electrodes 115 and 116 is measured through a reflected impedance technique
in a constant current AC circuit that communicates with electrodes 115 and 116 via
transformers 120 and 121. A resistor R
1 (typically about 20K ohm) is selected for stability, e.g. to avoid positive feedback
due to phase shift from the boundary layer capacitance at the electrodes. The winding
ratio on transformer 120 is 1:1, and the winding ratio on transformer 121 is 25:1.
The circuitry isolates the AC excitation means and the measuring means from the electrodes,
avoiding direct connections, d.c. polarizing effects, and providing the ability to
function over a relatively large common mode voltage range at the electrodes. The
circuitry also provides a linear signal-to-resistivity relationship over a relatively
large range.
[0049] As shown more specifically in Fig. 12, a 900 hz constant voltage A/C source 118 is
connected to the drive coil of transformer 120. The other coil of transformer 120
is connected to electrode 115 of detector 69. Electrode 116 is connected through resistor
R
1 to the drive coil of transformer 121 to complete the loop 210 from which electrode
impedance is to be communicated to the constant current AC circuit. Transformer 121
provides feedback to maintain constant current in the impedance measuring circuit.
The resulting signal from the constant circuit, reflected impedance detecting circuitry,
is connected to multiplexer 183 via filtered output, full-wave rectifier 181, and
non-inverting amplifier 182. The following table provides values and part numbers
for the schematically illustrated components.
[0050] As also shown in Fig. 12, sodium electrode 41 and reference electrode 34 are connected
to differential amplifier 190 to provide a signal representative of [Na
+] to multiplexer 183. A selector 191 selects an input signal (e.g. from amplifier
190, amplifier 182, or other circuitry not shown) to be output, through filter 187
and analog-to-digital converter 188, to computer 130, an Intel SBC 80/10B computer
comprising an 8080A CPU microprocessor.
[0051] First the standardizing solutions E
A and E
B are circulated through the flow path, and computer 130 stores signals representing
their respective conductivities and [Na
+], as well as the H
A, H
B, and
Na
STD values. When values for R
A and R
B have been determined, together with the known HA, H
B, and Na
STD values, then the corrected sample hematocrit H
X* can be derived by measuring R
X and Na
X, using computer 130 to perform the above calculations. A suitable program in assembly
language for performing those calculations on the 8080A CPU microprocessor is included
as an appendix to this application. In the program the hematocrit value is referred
to as (H
ct). III. Operation
[0052] The analyzer is used to measure characteristics of a blood sample. After the apparatus
is turned on, the various heaters and blowers are allowed to equilibrate and pump
29 is activated to create suction through the sample flow path and reference solution
is pumped through reference line 67. In order to flush the flow path, the probe is
retracted by drive motor 22, so that its inlet opening 21 is positioned in the flush-fluid
chamber of septum assembly 24. Flush fluid therefore is drawn through the flow path
and out the waste outlet 28, cleaning the flow path.
[0053] When the analyzer is idle, pump 29 is controlled to maintain a gas/liquid interface
at detector 103, thereby maintaining the electrolyte and pH electrodes in a liquid
environment while maintaining the PC0
2 and P0
2 electrodes in a gas environment.
[0054] To standardize the electrodes the probe inlet is introduced sequentially, under the
control of computer 130 and motor 22, into each septum assembly cavity; with the probe
positioned in a given cavity, the computer 130 controls pinch valve motor control
64 or solenoid valves 46 and 46' to open the desired standard fluid (liquid or gas)
to the septum assembly. Other standards are sealed by pinch valve 51 and solenoid
valves 46 and 46', to provide additional assurance against contamination. Standardizing
with liquids E
A,
EB, pH
A, and pH
B is accomplished by flowing a standard through the flow path and then holding it there
by appropriate control of pump 29 in response to liquid positions indicated by the
air detectors. Standardizing with G and G is accomplished by flowing those standards
along the flow path. Electrical signal values for each standard are recorded and stored
by storage means in computer 130 for later comparison with sample signal values. Valves
46 and 46' each comprise dual solenoid valves to allow a metered flow of standardizing
gas under the control of computer 130.
[0055] Standardization being complete, when analysis is required, the probe is fully extended
to draw sample solution through the flow path, without contamination from standards.
Signals representative of each measured sample characteristic are generated and transferred
to computer 130 for comparison with standard signals thus establishing a value for
each characteristic that is fed to output apparatus--i.e., C.R.T. display 12 and tape
printout 14. With the exception of the hematocrit measurement, the details of the
apparatus for generating standard and sample signals, for comparing those signals,
and for calculating values for sample characteristics are well known and need not
be repeated here.
[0056] Fig. 13 shows other aspects of the electronic components and their connection to
computer 130. Specifically, in Fig. 13, inputs to computer 130 are provided from keypad
16 and from multiplexer 183 via filter 187 and A/D converter 188. The computer provides
output to probe motor control 222, pinch valve motor controller 64, sample preheater
controller 160', air heater and blower controller 66', and solenoid valves 46 and
46'. Also, computer 130 provides output to CRT screen 12 and printer 14.
Other Embodiments
[0057] Other embodiments are within the following claim. For example, other blood components
or additional blood components can be sensed by the analyzer. Other electrolytes such
as [Cl ] can be used as a surrogate for hematocrit. In that case, suitable [Cl
-] concentrations of standardizing solutions E
A and E
B are 110 mM and 60 mM, respectively. In that case, 41 in Fig. 12A would be a [Cl
-] sensing electrode. In place of the electrode isolating circuitry described above,
the electrodes could be directly coupled to an AC conductivity measuring circuit with
a local ground (e.g. in the preheater).
1. A method for determining the hematocrit value of a blood sample by:
a) providing apparatus comprising a liquid flow path, means in the flow path for generating
an electrical signal representative of the electrical conductivity of liquid in the
path, and means in the flow path for obtaining an electrical signal representative
of the concentration of at least one ion species in liquid in the flow path;
b) introducing standardizing solution in the flow path having a known concentration
of said ion species and having a conductivity indicative of a known equivalent hematocrit
value, and obtaining a signal representative of said standardizing solution conductivity
and obtaining a signal representative of said known ion species concentration;
c) either before or after introducing the standardizing solution, introducing the
sample in the flow path and obtaining an electric signal representative of the sample
conductivity and an electric signal representative of the sample ion-species concentration;
and
d) deriving a tentative sample hematocrit value responsive to the sample conductivity
signal, with reference to said standardizing conductivity signal and to said known
standardizing equivalent hematocrit value; and
e) correcting said tentative sample hematocrit value with reference to said sample
and standardizing ion concentration signals and to said known ion concentration value.
2. The method of claim 1 wherein said method further comprises providing, from time
to time, an external validation of said apparatus by introducing a control solution
in said flow path, said control solution having a known ion species concentration,
and a conductivity representative of a known equivalent hematocrit level.
3. The method of claim 2 wherein said control solution equivalent hematocrit level
is within a physiologically normal range, and said control solution ion species concentration
is within a physiologically normal range.
4. The method of claim 2 wherein said control solution comprises an ion activity enhancing
agent.
5. The method of claim 2 wherein said agent is a polyol.
-6. The method of claim 5 wherein said polyol is selected from glycerol and polyalkyl
glycols.
7. The method of claim 1 or claim 2 wherein said conductivity obtaining step comprises:
a) providing electrodes in said flow path coupled to a constant current AC circuit
via a transformer;
b) applying an AC signal to said electrodes from said circuit via said transformer;
and
c) detecting impedance reflected in said AC circuit.
8. The method of claim 1 or claim 2 wherein said method comprises performing the following
steps in any order:
a) obtaining said electrical signals representative of standardizing conductivity
and standardizing ion concentration;
b) storing signals representative of said known standardizing equivalent hematocrit
value and said known standardizing ion concentration value;
c) obtaining said electrical signals representative of sample ion concentration and
standardizing ion concentration;
d) comparing said sample and said standardizing ion concentration signals with reference
to said stored known standardizing concentration value signal to derive a signal representative
of sample ion concentration value;
e) comparing said sample and said standardizing conductivity signals with reference
to said stored standardizing hematocrit value signal to derive a signal representative
of a tentative sample hematocrit value;
f) correcting said tentative sample hematocrit value signal with reference to said
sample ion concentration signal and said stored standardizing ion concentration value
signal.
9. The method of either claim 1 or claim 2 wherein said ion species is Na+ or Cl-.
10. Apparatus for determining hematocrit value in a blood sample comprising: 1) means
for providing a fluid flow path; 2) means in said flow path for providing an electrical
signal representative of the conductivity of liquid passing along said flow path;
3) means in said flow path for providing a signal representative of the concentration
of an ion species in liquid passing along said flow path; 4) means for introducing
said blood sample into said flow path to obtain a signal representative of sample
conductivity and of sample ion species concentration; 5) means for introducing into
said flow path a standardizing solution having a known concentration of an ion species
and having a conductivity representative of a known equivalent hematocrit value; 6)
means for deriving a signal representative of a tentative sample hematocrit value
from the sample conductivity signal, with reference to the standardizing conductivity
signal and to the standardizing equivalent hematocrit value; and 7) means for correcting
said tentative sample hematocrit value with reference to said standardizing and sample
ion concentration signals and to said known standardizing ion concentration.
ll. The apparatus of claim 10 wherein said apparatus comprises means for storing either
said sample or said standardizing conductivity signal, and means for comparing said
conductivity signals with reference to said known standardizing equivalent hematocrit
value to generate a signal representative of said tentative sample hematocrit value.
12. The apparatus of claim 11 wherein said means for correcting said tentative sample
hematocrit value signal comprises means for storing either said standardizing or said
sample ion concentration signal and comparing said concentration signals with reference
to said known standardizing ion concentration value.
13. The apparatus of claim 10 wherein said apparatus comprises at least two standardizing
solutions, each of which has a conductivity indicative of a known equivalent hematocrit
value and a known ion concentration.
14. The apparatus of claim 10 wherein said ion species is Na or Cl-.
15. The apparatus of claim 10 wherein said conductivity measuring means comprises
electrodes in said flow path, a constant current AC circuit coupled to said electrodes
via a transformer, and means for detecting reflected impedance in said AC circuit.
16. The apparatus of claim 15 wherein said conductivity signal-generating means comprises:
1) a first transformer for coupling said AC circuit to said electrodes; 2) a second
transformer for maintaining constant current in said AC circuit; and 3) means establishing
a loop, connected between said electrodes, comprising means connected in said loop
between said electrodes and said second transformer to compensate for inherent capacitance
at the electrode/sample interface.
17. The apparatus of claim 16 wherein said apparatus comprises an ion-species sensitive
electrode positioned in said flow path and connected via an electrical circuit to
the input of a multiplexer, said impedance detecting means also being connected to
the input of said multiplexer, said multiplexer having an output means connected via
an analog-to-digital converter to a means for storing and comparing signals, and to
said means for correcting sample conductivity.
18. A control solution kit for evaluating apparatus that determines a tentative level
for the hematocrit of a blood sample by determining the sample conductivity and correcting
said tentative level with reference to a sample ion species concentration level, said
kit comprising an aqueous solution comprising said ion species and an ion activity
enhancing agent, said solution having a known concentration of said ion and a known
equivalent hematocrit value.
19. The kit of claim 18 wherein said ion species is Na or Cl-.
20. The kit of claim 19 wherein said agent is a polyol.
21. The kit of claim 19 wherein said polyol is selected from glycerol and polyalkyl
glycols.
22. The kit of claim 18 wherein said ion concentration and said equivalent hematocrit
level are within physiologically normal ranges.